Process for preparing aluminum surfaces prior to plating



United States Patent 3,287,179 PROCESS FOR PREPARING ALUMINUM SURFACES PRIOR TO PLATING Theodore Frank, 6309 Bay Parkway, Brooklyn, N:Y.,

and Murray Goldwasser, 75-55 181st St, Flushing,

No Drawing. Filed Jan. 4, 1963, Ser. No. 249,318

4 Claims. (Cl. 1486.21)

This invention relates to a method of treating aluminum surfaces, and more particularly to a method of preparing aluminum surfaces for plating.

In the past, it has been found necessary to treat the surfaces of aluminum and aluminum alloys prior to plating with other metals such as nickel. These treatments generally involve the use of large quantities of dangerous chemicals and caustic soda, as well as other materials, which provide numerous disadvantages. Caustic soda exudes noxious fumes and has a tendency to spray, splash, sputter and burn any clothing or skin with which it comes in contact. In addition, the processes utilizing caustic soda require additional tanks, equipment, air ducts, guard facilities and safety installations to protect the personnel, as provided by local ordinances. The caustic soda process is also expensive and involved, since it generally requires between and process steps. The results of the caustic soda process are, furthermore, not uniform, since the coatings produced, using these treatments, are not strongly adherent and can contain peels, cracks or bubbles.

Generally, it has been necessary to apply two undercoatings, namely zinc and then copper, to the aluminum surface prior to coating with the desired surface metal. This procedure, which commercially requires some 12 to 21 process steps, involves treating the aluminum in a copper cyanide and sodium cyanide bath, then treating in a bath composed of copper cyanide, sodium cyanide and caustic soda. The above-mentioned chemicals used in this process are expensive, exceedingly dangerous to use and the cyanide compounds can be used only under strict government regulations. In addition, because of their position in the electro-chemical series, zinc, copper or any combination thereof aggravates the corrosion of aluminum and aluminum alloys, during use of the plated material.

It is therefore an object of the present invention to provide a new and improved process of treating aluminum and aluminum alloys prior to coating the same with metals.

It is another object of the present invention to provide a new and improved metal treating composition.

It is another object of the present invention to provide a simplified and inexpensive method of treating aluminum and aluminum alloys prior to metal plating.

It is another object of the present invention to provide a safe method of treating aluminum and aluminum alloys prior to metal plating.

It is another object of the present invention to provide 3,287,179 Patented Nov. 22, 1966 salt solution to treat the surface of the aluminum or aluminum alloys to provide an alkali metal deposit and an oxide coating thereon. After such treatment, according to the present invention, the aluminum is then rinsed and deoxid zed by washing in any common deoxidizer such as nitric acid. The deoxidizer is then rinsed from the aluminum which can then be deposited into any conventional plating tank where the desired metal coating, such as nickel, is directly deposited thereon.

It is preferred in the practice of this invention to maintain the proportion of ingredients of the composition of the oxidizing bath within the following range:

Percent An aqueous solution of a salt of an alkali metal An aqueous solution of a salt of a chromic acid 1.9 to 20 An aqueous solution of a salt of a perchloric or chloric acid 3 to 26 An aqueous solution of sodium lauryl sulphate .05 to .5

In accordance with the method of the present invention, the above solution is heated to between 150 to 200 F., and the aluminum or aluminum alloy is immersed in the aqueous solution for a period of from two to ten minutes, depending on the type of aluminum or aluminum alloy being treated. It has been found that over-immersion in the bath Will not damage the article being treated; however, an immersion of greater than ten minutes has been found to provide no further treatment. This treatment provides an alkali metal deposit and an .oxide coating of the desired properties on the surface of the aluminum. The aluminum is then withdrawn from the aqueous solution and rinsed in water.

After the water rinse, the treated aluminum or aluminum alloy is deposited or submerged in any common deoxidizer which provides a clean metal receptive surface. It has been found that nitric acid in aqueous solutions of from 10% to 50% are satisfactory as deoxidizers. Other useful deoxidizing solutions are aqueous solutions of from 2% to 10% of sodium dichromate and 1% to 5% solutions of chromic acid. The preferred deoxidizer, however, is an aqueous solution of 25% nitric acid. After the deoxidation step, the aluminum is rinsed in Water and then deposited in any conventional plating tank without being subjected to any further process steps or any other coating procedure.

The invention is further illustrated by the following exampleswhich are not to be considered as in any way limiting the scope thereof.

Example One 5 parts of sodium chloride, NaCl a process of treating aluminum and aluminum alloys prior to metal plating which results in reduced time and labor.

It is still another object of the present invention to provide a method of treating aluminum and aluminum alloys, in conjunction with metal plating, which consistently provides flat adherent metal coatings free of peels,

cracks and bubbles.

It is yet another object of the present invention to provide a method for the treatment of aluminum and aluminum alloys which eliminates the necessity of undercoating the desired metal with an intervening coating of zinc and copper.

Other objects and the nature and advantages of the instant invention will be apparent from the following description.

The present invention involves the use of an aqueous 2 parts of potassium dichromate, K Cr O 3 parts of sodium perchlorate, NaClO of a part of sodium lauryl sulphate parts of water 10 parts by weight of potassium chloride, KCl 5 parts of potassium dichromate, K Cr O 20 parts of magnesium perchlorate, Mg(ClO of one part of sodium lauryl sulphate 65 parts of water The components are mixed together to form an aqueous solution which is heated to 175 F.

An aluminum object is treated as described in Example One, except that the dwell time in the oxidizing solution is reduced to three minutes. The nickel coating resulting from this process is found to be strongly adherent and free of peels, cracks and bubbles.

Example Three 20 parts by weight of potassium nitrate, KNO 15 parts of ammonium chromate, (NH CrO parts of ammonium perchlorate, NH ClO A of a part of sodium lauryl sulphate 60 parts of water The components are mixed to form an aqueous oxidizing solution which is heated to 175 F. An aluminum object is treated as described in Example One except that the dwell time in the oxidizing solution is reduced to seven minutes. The nickel coating of the present method is found to be strongly adherent and free of cracks, bubbles and peels.

Example Four 45 parts by weight of sodium carbonate, Na CO 20 parts of ammonium dichromate, (NHQ Cr O- 25 parts of magnesium perchlorate, Mg(ClO /2 part of sodium lauryl sulphate 40 parts of water The components are mixed to form an aqueous oxidizing solution which is heated to 150 F. An-aluminum object is treated as described in Example One, except'that the object is removed from the oxidizing solution after a dwell time of four minutes. The nickel coating resulting from the process is found to be strongly adherent to the aluminum object and free of cracks, bubbles and peels.

Example Five 25 grams of sodium carbonate, Na CO grams of sodium chromate, Na CrO 20 grams of sodium chlorate, NaClO /2 gram of sodium lauryl sulfate 1 liter of water Example Six 20 grams of sodium carbonate, Na CO 5 grams of sodium dichromate, Na Cr O 10 grams of sodium perchlorate, NaClO 1 gram of sodium lauryl sulfate 1 liter of water Here the process, according to Example One, was carried out and the result was perfect adhesion without cracks, bubbles or peeling.

Example Seven 30 grams sodium carbonate, Na- CO 20 grams sodium chromate, Na Cr0 30 grams sodium chlorate, NaClO /2 gram sodium lauryl sulfate 1 liter of water The process, according to Example Five, was carried out and the result was perfect adhesion without cracks, bubbles or peeling,

4 Example Eight Objects produced by the processes of Examples One through Seven are compared with two samples of commercially purchased nickel coated aluminum. All nine samples have a nickel coating of about .0005 inch thickness. The nine samples are placed in an electric furnace for five minutes at a temperature of 350 F. The samples of Examples One through Seven show no blistering, peeling or fracture of the nickel plate. The first commercial example, however, shows blistering and peeling, whereas the second shows fracture of the nickel coating.

Example Nine Objects of Examples One through Seven are subjected to a bend test of approximately 180 and back and showed found most useful, however, are Na CO K CO and.

small percentages of NaOH or K-OH.

Examples of chromic acid salts are K Cr O K CrO Na Cr O- Nap GTO; and (NH4)2CI'2O7. Those materials found most useful, however, are Na cr O- Na CrO K Cr O K CrO and chromic acid itself.

Examples of salts of perchloric acid are M (ClO NH CIO KClO and NaClO However, the salts of chloric acid are preferred since these are safer, e.g. NaClO and KClO If desired, each group may be formed of a mixture of the salts from its group.

As a wetting agent or surface active agent, sodium lauryl sulphate is preferred. However, other surface active agents may be used, such as, for example, sodium alkylnaphthalene sulfonate or an alkylphenyl polyethylene glycol ether.

The present invention provides a great advance in the plating art, since no noxious fumes are evolved, no danger is present to the operators working in or about the process, and the invention is much faster than the previously used system. Furthermore, the present system does not 'require a large capital investment and the results are consistently uniform. While the methods of the prior art required 12 to 21 steps, the present invention can be carried out in as little as five or six steps requiring as little as four minutes total bath time. In addition, the chemicals used in the present invention are very inexpensive. in comparison with the chemicals used in the prior art.

The new system does not require costly control of each step by the operator. An operator needs no special training or knowledge. The present process does not affect the lustre or prepolished surface of the aluminum and aluminum alloys and in fact actually assists in maintaining such qualities. The nickel surface of the present invention is directly receptive to all other metals without any necessity of undercoating the nickel.

It will be obvious to those skilled in the art that various changes may be made without departing from the scope of the invention and therefore the invention is not limited to what is described in the specification but only as indicated in the appended claims.

What is claimed is:

1. A composition of matter comprising by weight an aqueous solution of 5 to 50% of a salt of an alkali metal, 1.9 to 20% of a salt of a chromic acid, 3 to 26% of a third salt of an acid selected from the group consisting of chloric and perchloric, and a small percentage of a wetting agent.

2. A composition in accordance with claim 1, wherein said third salt comprises a salt of perchloric acid.

3. The method of metal plating comprising: (1) contacting an aluminum containing surface for about two to ten minutes with a heated aqueous solution of 5 to 50% of a salt selected from the group consisting of sodium carbonate and potassium carbonate, 1.9 to 20% of a salt of a chromic acid selected from the group consisting of sodium and potassium chromates and dichromates, 3 to 26% of a salt selected from the group consisting of sodium chlorate and potassium chlorate, and a small percentage of a Wetting agent; (2) contacting said surface with a deoxidizer and. (3) plating said surface with another metal.

4. A method in accordance with claim 3, wherein said aqueous solution consists essentially of approximately 20 parts sodium carbonate, 10 parts sodium chromate, 20 parts sodium chlorate and /2 part sodium lauryl sulfate Wetting agent.

References Cited by the Examiner UNITED STATES PATENTS Guthrie 148-62 X Kappes.

Zurbrugg 148-6.2 X Thompson 148-62 Blackmun et al. 148-62 X Shawcross 148-62 X Waring 148-627 X Mason 148-627 X Thomson 1486.2X Shaw 148-627 X ALFRED L. LEAVITT, Primary Examiner.

R. S. KENDALL, Examiner. 

1. A COMPOSITION OF MATTER COMPRISING BY WEIGHT AN AQUEOUS SOLUTION OF 5 TO 50% OF A SALT OF AN ALKALI METAL, 1.9 TO 20% OF A SALT OF A CHROMIC ACID, 3 TO 26% OF A THIRD SALT OF AN ACID SELECTED FROM THE GROUP CONSISTING OF CHLORIC AND PERCHLORIC, AND A SMALL PERCENTAGE OF A WETTING AGENT. 